The invention relates to machines and methods for generating longitudinally curved tooth spaces in bevel and hypoid gears. In particular, the invention relates to computer numerically controlled bevel and hypoid gear generating machines and methods whereby a reduced number of movable machine axes are provided for setup and operation.
In the context of the present invention, the phrase "bevel and hypoid" is understood to mean either or both types of gears because of a lack of agreement in the art concerning the use of either term (bevel or hypoid) as generic to the other. Accordingly, whether bevel gears are considered a specific type of hypoid gears or visa versa, the present invention contemplates machines and methods for forming longitudinally curved tooth surfaces of either or both gear types.
Machines for generating bevel and hypoid gears are generally arranged to carry cutting or grinding tools in a manner which permits the tools to represent a mating gear member in mesh with a work gear being produced. For example, it is understood in the art that if both members of a mating pair of work gears are separately manufactured with tools representing complementary theoretical generating gears in mesh with each work gear member, the manufactured work gears will mesh properly with each other.
According to usual practice, tooth surfaces of one or both members of a mating work gear pair are manufactured by a relative rolling process with a tool as though the work gear were in mesh with a theoretical generating gear represented by the tool. Such generating processes, however, are quite time consuming and it is often preferred to generate only one member of a gear pair. For example, many bevel and hypoid gear pairs used in automotive applications are manufactured according to a process in which tooth surfaces of a first gear member (usually a ring gear) are formed without generation (i.e., the tool is oriented to represent tooth surfaces of a stationary theoretical gear and work gear tooth spaces adopt the form of tooth represented by the tool) and the tooth surfaces of the other gear member (usually a pinion) are generated using a tool which is oriented to represent tooth surfaces of the first formed gear member in mesh with the other gear member.
For purposes of economy, two types of gear making machines have evolved for producing different members of work gear pairs where only one member of the pair requires generation. Those machines which are arranged to represent the rolling motion of a theoretical generating gear in mesh with a work gear are referred to as "generating machines" and those machines which are arranged to represent a stationary theoretical gear are referred to as "non-generating machines." Generating machines are required to impart additional motions and, therefore, are much more complicated and expensive than non-generating machines. A significant cost savings is associated with the use of less expensive non-generating machines to produce one member of each gear pair. Generating machines may be used to manufacture non-generated tooth gears but non-generating machines do not include required settings and controls to manufacture generated tooth surfaces.
Typical bevel or hypoid gear generating machines include a machine base and separate supports resting on the base for mounting a work gear and a rotating tool. The tool support is arranged to carry a rotary tool in a manner which represents a theoretical generating gear positioned to mesh with the work gear. A machine cradle is journaled in the tool support so that its axis of rotation represents the axis of the theoretical generating gear. A rotary tool, having stock removing surfaces which represent one or more teeth in the theoretical generating gear, is supported on the front face of the cradle. In particular, the rotary tool is mounted on a tool spindle which is journaled in a tilt mechanism carried by the cradle. The tilt mechanism is used to adjust the angular position of the rotary tool axis with respect to the cradle axis so that the stock removing surfaces of the tool are oriented to appropriately represent the position of gear teeth on the theoretical generating gear.
The work gear support generally includes means for adjusting the mounting position of the work gear so that the work gear will fit into mesh with the theoretical generating gear represented by the tool support. The work gear is journaled for rotation in the work support and means for rotating the work gear interconnect with means for rotating the machine cradle so that the work gear may be rotated in a timed relationship with the rotation of the cradle. Tooth sides are generated in the work gear by imparting a relative rolling motion between the tool and work gear as though the work gear were in mesh with another gear member (i.e., the theoretical generating gear) having an axis of rotation coincident with the machine cradle axis and mating tooth surfaces represented by the stock removing surfaces of the tool.
The rotary tool may be arranged to represent a single tooth in a generating gear or may include a number of stock removing surfaces which are specially positioned on the tool body for timed rotation with the work gear to represent a generating gear with a plurality of teeth.
"Intermittent indexing" processes are associated with the use of a rotary tool which is designed to represent a single tooth of a generating gear. According to the known practice of intermittent indexing, each successive tooth space is formed one at a time until all of the desired number of tooth spaces are formed in the work gear. For example, generating motions in which the tool is rotated about the cradle axis in a timed relationship with work gear rotation are performed independently for each tooth space.
A second well known gear making process, known as "continuous indexing", uses a rotary tool arranged with a number of stock removing surfaces which are positioned on a tool body to represent a plurality of teeth on the generating gear. According to this known practice, the work gear is rotated in a timed relationship with the rotation of the tool so that all of the tooth spaces in the work gear are collectively formed. Required generating motions are superimposed on this timed relationship so that an additional amount of work gear rotation is imparted in a timed relationship with machine cradle rotation. Since all of the work gear tooth spaces are treated collectively, only a single rotation of the tool about the cradle axis through the space of one tooth of the theoretical generating gear is required to completely generate all work gear tooth surfaces.
A discussion of the basic machine requirements of bevel and hypoid gear manufacture is found in Chapter 20 of Gear Handbook, The Design, Manufacture, and Application of Gears, Darle W. Dudley, Editor, Copyright 1962 by McGraw-Hill, Inc., Library of Congress Catalog Card Number: 61-7304. Pages 1 through 11 of the cited chapter entitled "Bevel- and Hypoid-Gear Manufacture" are hereby incorporated by reference for purposes of indicating the background of invention and illustrating the state of the art.
For purposes of additional background, it may be appreciated that for a number of years, advances in the computer and electronics industry have been routinely applied to machine tools. In fact, most state-of-the-art machine tools now include some sort of computer control. Such machines are referred to in the industry as computer numerically controlled machines or "CNC" machines. It is well known, for example to use computers to control both machine operation and setup. Computers also enable a series of machines performing separate functions to work together in a system to perform many different operations on work pieces and to produce a number of different work pieces without requiring substantial manual intervention.
Although conventional bevel and hypoid type gear generating machines have been recently fitted with computer controls, mainly for monitoring and controlling machine operation, much of the set up of these machines still requires manual intervention. For example, U.S. Pat. No. 3,984,746 discloses a "master-slave" servo-system for replacing certain gear trains in a conventional bevel and hypoid gear generating machine which control relative machine motions during use. However, much of the setup of the modified machine still requires substantial manual intervention.
Conventional bevel and hypoid gear generating machines (i.e., those described in the above-referenced Gear Handbook) require nine or more machine settings (also known as "setup axes") for appropriately positioning the tool with respect to the work gear. These settings include: (a) an angular setting of the cradle, (b) three angular settings of the tilt mechanism, (c) a rectilinear feed setting between the tool and work supports, (d) a rectilinear setting of work gear height above the machine base, (e) an angular setting of the work gear axis, (f) a rectilinear setting of the work gear along its axis, and (g) for certain cutting methods, relative settings of the rotational positions of the tool and the work gear. These settings are difficult to make to required accuracy and are time consuming. Most of these settings are accomplished manually because the large number of settings and their often congested locations render computer control of these settings extraordinarily complex and/or prohibitively expensive.
For example, known tool tilt mechanisms on bevel and hypoid gear generating machines are associated with a number of particularly difficult settings. These settings are made to incline and orient the tool axis with respect to the cradle axis so that the stock removing surfaces of the tool are positioned to appropriately represent tooth surfaces of the theoretical generating gear. Three coordinated settings known in the art as "eccentric angle", "swivel angle" and "tilt angle" are usually required for this purpose.
The tool drive which acts through the tilt mechanism also involves an extraordinary amount of complexity. This drive is required to impart rotation to the tool at variable angular orientations and positions with respect to the cradle axis. Thus, both the complex settings of the tool tilt mechanism and the tool drive at variable orientations take place within the space of the machine cradle which is itself rotatable.
Accordingly, machine cradles tend to be quite large and cumbersome. The diameter of the theoretical generating gear represented by the tool support is also substantially determined by the diameter of the machine cradle. For example, a 60 centimeter diameter cradle may be required to support a rotary tool in position to represent tooth surfaces of a 30 centimeter diameter theoretical generating gear. Machine cradles are difficult to manufacture and mount to required precision, and account for a significant portion cf the size, weight and cost of conventional generating machines. It has been proposed on occasion to replace the customary machine cradle with a pair of rectilinear slides. For example, FIG. 20-7 on page 8 in the incorporated chapter of "Gear Handbook" illustrates this possibility for non-generating machines. As explained therein, non-generating machines require positioning of a tool axis with respect to a work gear axis in the manner of a machine cradle but they do not require any motion of the tool axis equivalent to cradle rotation. Thus, it may be readily understood that the just-mentioned slides may be used to move a tool axis to the same position otherwise effected by a cradle in non-generating machines. This general concept has also been proposed for bevel and hypoid generating machines in SU, A, No. 724287 (V. A. KONDYURIN) and DE, A, No. 36 43 967 (YUTAKA SEIMITSU KOGYO K.K.). In the proposed machines disclosed in these patents, the customary machine cradle is replaced by a pair of rectilinear slides which can be controlled to move the tool axis along an arcuate path corresponding to cradle rotation during generation.
However, neither of these proposed generating machines suggests any means for inclining the tool axis with respect to their intended representation of the customary cradle axis. In fact, even if a known tool axis tilt mechanism were to be added to either of the proposed machines, the arcuate translation of an inclined tool axis along the rectilinear slides of the proposed machines would not reproduce the rotational motion of the inclined tool axis about the cradle axis of a conventional machine. In other words, translation of an inclined axis about another axis to which it is initially inclined, is not the same as rotation of the inclined axis about another axis.
Thus, neither of the proposed generating machines disclosed in the patents just referred to above is appropriate for manufacturing the variety of gears traditionally produced by conventional bevel and hypoid generating machines which utilize large machine cradles and complex tilt mechanisms for appropriately positioning and operatively engaging a tool and work gear. Furthermore, even when generating without any provision for tool axis tilt, neither of the proposed machines appears to account for the change in angular position of the tool about its axis which should accompany their respective translational representations of cradle axis rotation, and the lack of such a change in angular position would undesirably affect the required timed relationship between tool and work rotations during continuous indexing operations.
Another known alternative configuration for a computer controlled bevel and hypoid- gear generating machine operates according to a different method and is disclosed in U.S. Pat. No. 4,565,474. This machine uses a CNC system for controlling machine axes for purposes of both setup and operation. However, even for the generation of only one of the flanks of longitudinally curved tooth spaces, the machine requires a large number of movable axes for setup and operation. Of these required axes, the most difficult and expensive ones to control are rotational (or pivot) axes for angularly moving the tool and work gear axes with respect to each other. For the generation of a single flank of longitudinally curved tooth gears, the machine includes a first axis for pivoting the tool with respect to the work gear, a second axis for pivoting the work gear with respect to the tool, and a tool tilt mechanism (e.g., the mechanism of U.S. Pat. No. 4,370,080) for inclining the tool with respect to a third axis. These three just-named axes are in addition to axes for rotating the tool and work gear, and also in addition to three rectilinear axes of relative movement between the tool and work gear. If the other flank of the tooth spaces is to be generated simultaneously, a second tool is required along with further additional axes for rotating and pivoting the second tool, two more rectilinear axes for moving the second tool with respect to the work gear, and presumably, a second tool axis tilt mechanism.
The machine disclosed in U.S. Pat. No. 4,565,474 operates according to limit conditions in which a single generating line of constant shape (e.g., analogous to a straight or curved rigid wire) is used to generate ruled tooth surfaces. The generating line is defined by an intersection of the stock removing surface swept by a rotating tool with a plane of action which comprises a locus of points of contact between the tool and work gear. The tool penetrates the plane of action at a fixed depth, and the constant shape generating line in the plane of action is rotated about an axis perpendicular to the plane in a predetermined relationship with rotation of the work gear in a manner such that the plane of action rolls together with a base cone surface of the work gear. Generated thereby in the plane of action is a ruled tooth surface defined by relative movement of the constant shape generating line with respect to the work gear.
These limitations which define the method of U.S. Pat. No. 4,565,474 prevent the disclosed machine from duplicating tooth surfaces now being provided by conventional machine motions which represent the rolling motion of a theoretical generating gear in mesh with the work gear. Conventional machine motions provide for rotating the tool about the axis of a theoretical generating gear (i.e., cradle axis) which defines a relative path of movement between the tool and work gear that is disposed angularly with respect to the plane of action between them. Contact between the tool and work gear progresses along various points on the tool's stock removing surface which is itself inclined to a normal (perpendicular) of the plane of action by a pressure angle corresponding to the generating gear tooth surface represented by the tool. Thus, the tool according to conventional methods is moved along a path inclined to the plane of action, penetrating the plane of action at changing depths along a tool surface also inclined to a normal of the plane of action, thereby generating tooth surfaces, not by a single generating line of constant shape, but rather, by an enveloping process in which the generated tooth surfaces are defined by the stock removing surface of the tool and its relative motion with respect to the work gear. Also, since the stock removing surfaces of tools used in conventional machines are inclined to the plane's normal and moved angularly with respect to the plane of action, it is possible to simultaneously generate both flanks cf tooth spaces in a work gear with a single tool arranged to represent both flanks of a generating gear tooth.
Another important consideration relating to the generation of longitudinally curved tooth bevel and hypoid gears is the determination of appropriate setup and operating parameters for such machines. Because of the complexity of tooth surfaces formed by conventional bevel and hypoid generators, such tooth surfaces can only be exactly defined geometrically by the machine motions which are used to produce them. That is, although certain general parameters of gear design may be specified, e.g., tooth numbers, pitch angle, etc., the equations which are used to define bevel and hypoid tooth surfaces are the motion equations of generating machines. Since tooth surfaces are not defined independently of machine motions, the design of tooth surfaces is often an iterative type of process known in the art as "development."
Much know-how has been accumulated, particularly in the form of computer software, for developing tooth surfaces by appropriate adjustment of the operating parameters of conventional bevel and hypoid generating machines.
It should be appreciated that much of this extremely valuable, painstakingly accumulated know-how would be of little use in controlling the operation of non-conventionally configured machines which require different operating parameters for controlling machine motions. Instead, such alternatively configured machines would require new sets of formulas and other know-how to determine appropriate machine settings and operating parameters for producing known gear tooth geometry and mating characteristics. For example, a machine described in an article in the July 1983 issue of American Machinist magazine on pages 85 through 88, entitled "Generating Gears Via Software," describes such a method for determining the operating parameters of a non-conventionally configured machine. (The machine referred to in the article appears related to the machine disclosed in U.S. Pat. No. 4,565,474.)
It will be appreciated that the determination of such appropriate machine settings and operating parameters, based on general information of desired tooth geometry and mating characteristics, corresponds to the above-mentioned process of development utilized with conventional machines. While it may be relatively practicable to add software to a CNC machine for determining such required machine motions for producing easily defined straight tooth gears, the additional complexity of longitudinally curved bevel and hypoid gear teeth, particularly those which are produced by conventional generation, would require extensive and complicated developments that would add considerable cost and complexity to any CNC system which otherwise is primarily responsible for controlling machine motions along prescribed paths.
Further, little or no benefit would be derived from the large amount of existing know-how which relates such desired tooth geometry and mating characteristics to conventional machine settings. This is particularly true of "higher order" modifications which are expressed directly in terms of known machine motions or in terms of a theoretical generating gear.
In general, it is already known from U.S. Pat. No. 3,984,746 to incorporate computer numeric controls in conventional bevel and hypoid generating machines for automatically setting up and operating certain of their movable machine axes. However, the large number and congested locations of the conventional machine axes makes the application of computer controls to all of these axes especially complicated and expensive. Although it is also known from SU, A, No. 724287 and DE, A, No. 36 43 967 to replace the large and cumbersome machine cradle of conventional machines with a pair of rectilinear slides, neither of these two proposals discloses any means for inclining the tool axis with respect to the original cradle axis or for replacing the rotational function of the conventional machine cradle in so far as the latter rotates the inclined tool axis about the cradle axis. Further, the computer controlled gear generating machine of U.S. Pat. No. 4,565,474, which includes a much different configuration of machine axes, still requires a large number of controlled axes for generating longitudinally curved tooth gears. Also, the disclosed method of controlling these axes undesirably limited to the generation of ruled surfaces and does not benefit from accumulated know-how relating to development of tooth designs producible on conventional machines. Finally, none of the known art, considered separately or in combination, suggests any means whereby the conventional arrangement of a tool tilt mechanism carried on a machine cradle can be entirely replaced by computer controlled axes arranged in a more simplified configuration.